1. Field of the Invention
The present invention relates to a temperature indicating material, for used as a temperature indicator which is used for the temperature monitoring of vegetables and fruits, perishable foods and the like, and which undergoes color changes with temperature.
2. Discussion of the Background
As a temperature indicating material which undergoes color changes with temperature, thermochromic organic dye, for example, ethylene derivatives substituted with a fused aromatic ring such as spiropyrans bianthrone or dixanthylene, crystals of a metal complex salt composed of CoCl2xc2x72(CH2)6N4xc2x710H2O, combinations of an electron donating compound, an electron accepting compound and a polar organic compound and the like have conventionally been put on the market. Upon use of a temperature indicating label prepared from such a temperature indicating material, color changes with temperature are judged visually.
Roughly speaking, there are two types of temperature indicating materials, that is, a reversible type and irreversible type. In the former one, the color changes unlimitedly in response to a temperature increase or decrease, while in the latter one, the color which has once changed at a certain temperature does not show any further change.
The reversible type material is used for indicating a cooling level for drinking of beer or the like, or for visually attracting attentions to a temperature change.
The irreversible type material is used for the temperature monitoring of fruits and vegetables, or perishable foods, for example. The temperature indicating material which has stored its thermal history is disclosed in Japanese Patent Publication No. SHO 58-10709, wherein an ink having a changeable viscosity with temperature (for example, an ink which has a dye dissolved in oleyl alcohol) penetrates in a penetration material and the length of the ink penetrated in the material indicates how long it was exposed to a temperature not lower than a predetermined temperature. In addition, as another form, there is a temperature indicating material which has a plurality of layers formed on a label. These pluralities of layers are an ink layer, a separate layer, a porous layer and an indicator layer and they are stacked one after another. The separate layer is removed when the temperature monitoring is started, while the porous layer controls the penetration time of the ink from the ink layer until it reaches the indicating layer. For such a temperature indicator member, in case of a predetermined temperature lower than a room temperature, a storing device prior to the starting of temperature monitoring is important, it needs to form the separator or the like, so that it is expensive and in addition, has such a complex structure that a desired printing pattern cannot be adopted. Moreover, this indicator needs a cumbersome operation such as removal of the separator or the like when it starts on the temperature monitoring.
As an improved process, a process using a rewritable material is disclosed in Japanese Patent Application Laid-Open No. 197853/1996, wherein the glass transition temperature of the material is used as a predetermined temperature and temperature monitoring is effected by making use of the fact that a drastic change in the diffusion rate of molecules occurs at a temperature exceeding the glass transition temperature. The above method uses a temperature indicating material including an electron donating compound, an electron accepting compound, a reversible material causing reversible transformation between crystal and amorphous, or reversible transformation between phase separation and non-phase-separation, and a phase separation controller which changes phase separation speed between the electron donating compound or the electron accepting compound and the reversible material. The method defines the glass transition temperature of the composition system lower than room temperature so as to record the history of raising temperature as color change by crystallization of composition system. The method also introduces decolorize for initialized using a thermal head for heating and quenching the temperature indicating material. The heating and quenching mean instantaneous heat application by the thermal head. Thereafter the temperature indicating material shows temperature change in its environment using change color by temperature rising.
However, the rewritable material, as introduced in Japanese Patent Application Laid-Open No. 197853/1996, cannot be decolorized completely and the colorize progresses in low temperature. Thus, the temperature indicating material disclosed in Japanese Patent Application Laid-Open No. 197853/1996 has drawback that large difference been color densities cannot he obtain in response to temperature change.
The phase separation controller disclosed in Japanese Patent Application Laid-Open No. 197853/1996 improves colorize speed around melting point or higher than melting point. Thus, the phase separation controller cannot change setting temperature of the electron donating compound, the electron accepting compound and the reversible material in condition of under room temperature. That is, the melting point of the phase separation controller is set under room temperature, in condition that the setting temperature is lower than room temperature, since colorize speed should be promoted around setting temperature. Thus, the glass transition point becomes too low, so that storage stability cannot be kept sufficiently. Accordingly, the phase separation controller disclosed in Japanese Patent Application Laid-Open No. 197853/1996 is not proper for the temperature indicating material to be used in condition lower than room temperature.
Further, the material having melting point lower than room temperature is not proper for dispersion in a binder resin or the like, since the material is liquid or half solid (border state between liquid and solid). Such material can be dispersed in the binder resin when the system including the electron donating compound, the electron accepting compound and the reversible material and the phase separation controller is formed as micro-capsule, but cost for such system will be increased.
Alicyclic alcohol not having long normal chain like cyclododecanol as disclosed in Japanese Patent Application Laid-Open No. 197853/1996 is generally not proper for use in the phase separating controller. Alicyclic alcohol improves decolorize characteristic when the alicyclic alcohol is used as a temperature characteristic controller. The thermochromisnm controller including alicyclic alcohol can be delaying colorize at low temperature and promoting colorize at high temperature, so that the thermochromisnm controller operates sensitively to temperature. However, mutual action between the temperature characteristic controller including alicyclic alcohol and the electron accepting compound (developer) is large, and alycyclic alcohol prevents the mutual action between the electron donating compound (electron donor organic substances) and the electron accepting compound (developer), so that the colorize density will be poor as compared with case without the temperature characteristic controller.
Accordingly, an object of the present invention is to provide a temperature indicating material to be capable of executing temperature monitoring operation in the environment under room temperature without various drawbacks.
The object of the present invention is achieved by the novel temperature indicating material of the present invention.
According to the novel temperature indicating material of the present invention, it includes an electron donating compound; an electron accepting compound; a reversible material causing reversible transformation between crystal and amorphous, or reversible transformation between phase separation and non-phase-separation, with respect to a part or all of the composition system; and a temperature characteristic controller. The temperature characteristic controller is solid-state in room temperature. At least a part of the temperature characteristic controller dissolves in the electron accepting compound, the reversible material, or the electron accepting compound and the reversible material so as to change speed of the reversible transformation between crystal and amorphous, or speed of the reversible transformation between phase separation and non-phase-separation, with respect to the position system, by its reversible transformation between crystal and amorphous, or speed of the reversible transformation between phase separations and non-phase separation. The ratio between one mutual action and another mutual action, one mutual action is the mutual action between the electron donating compound and the electron accepting compound after phase separation with respect to the composition system in accordance with change of temperature and time after initialized by heating and quenching, another mutual action is the mutual action between the electron donating compound and the electron accepting compound before initialized, is same or more to the same kind of ratio with respect to the temperature indicating material not including the temperature characteristic controller.